The present invention relates to the formation of plastic articles having multilayered walls by coinjection of two or more thermoplastic materials wherein the starting materials can if desired have substantially different optimum processing temperatures.
The simultaneous or sequential injection (coinjection) of two or more resins into a mold cavity to develop layered wall structures is well known as evidenced by the disclosures of U.S. Pat. Nos. 4,497,621, 4,518,344, 4,525,134, 4,526,821, 4,609,516 and 4,657,496.
These disclosures, while providing separate flow paths for the respective resins, do not show or suggest a method for maintaining each resin at its optimum processing temperature during transit from an extruder or other source to the mold cavity.
The maintenance of processing temperatures of each individual resin is especially important when the optimum processing temperature of one resin causes degradation of a second resin or vice versa.
For example, it is frequently desirable to mold a layered wall structure for a hollow article or a preform where coinjection materials include ethylene vinyl alcohol copolymer (EVOH) which process most satisfactorily at temperatures ranging from 400.degree. to 440.degree. F. and polyethylene terephthalate (PET) which processes best at temperatures ranging from 500.degree. to 550.degree. F. As stated previously, failure to maintain these optimum temperatures individually leads to degradation of one resin or the other resulting in defective product.
Various art references describe the process of sequential coinjection including U.S. Pat. Nos. 4,710,118, 4,497,621, 4,657,496, 4,808,101 and 4,701,292.
In addition, art references describe insulating various components in a sequential coinjection device so that the resins can be processed at their optimum temperatures, see U.S. Pat. Nos. 4,717,324, 4,775,308, 4,863,665 and 4,931,234.
It is desirable to develop an apparatus to coinject two or more resins into a mold cavity through the same orifice while minimizing thermal degradation, especially to provide separate temperature control of the resins in the hot runner area and to minimize the volume and residence time of one of the resins in the nozzle area so as to minimize its exposure to thermal degradation.